Ruthenium olefin metathesis catalysts for transformations of renewables.

Olefin metathesis is a metal-catalyzed transformation through which pairs of carbon-carbon double bonds are rearranged allowing simple molecules to easily transform into more complex and precious compounds. The synthesis of a huge number of natural products, polyfunctional biologically active products as well as polymers by olefin metathesis has been achieved to date.1 As petrochemical resources become increasingly scarce and expensive, much research attention has been focused on renewable resources as green alternatives for producing chemicals. Catalytic olefin metathesis is a powerful tool to transform bio-sourced structural motifs, containing carbon–carbon double bonds (e.g. phenylpropenoids, terpenes, unsaturated fatty acid esters), in valuable compounds for the chemical industry. In this view, easy-handling, stable ruthenium-based catalysts supported by N-heterocyclic carbene (NHC) ligands are considered as excellent candidates. The appropriate choice of the NHC ligand architecture is one of the key issues for the development of highly active and selective catalysts.2 The introduction of unsymmetrical NHCs (uNHCs), modulated by different substituents at the nitrogen atoms of the NHC ring, has led to important effects on the reactivity and selectivity of the resulting catalysts, thus allowing for significant advancements in some challenging or specific olefin metathesis reactions. With the aim of increasing catalyst efficiency in olefin metathesis reactions involving renewable substrates of both academic and industrial relevance, herein we report the synthesis and the catalytic behaviour in specific metathesis applications of new ruthenium complexes bearing uNHCs combining different backbone configurations (syn or anti) with N-alkyl/N-aryl substituents of variable bulkiness.